I believe dieselnut was considering swappin a diff into a 300D. 1985 300D speedometers are conventional meaning a diff from an 85 will work all the way down to a 79, further back perhaps.
I have them in my 123's. I love that 2.88 diff! They are NOT noisy!
Definitely worth looking for one if you have a turbo engine that is strong. The SD is another story though but I thought the thread was about a 300D?

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!